A known pressure sensor as shown in U.S. Pat. No. 4,875,135, assigned to the assignee of the present invention, the subject matter of which is included herein by this reference, comprises a capacitive pressure sensing transducer having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a ceramic base and having metal coatings deposited on respective opposing surfaces of the diaphragm and base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A connector body of electrical insulating material is fitted over the electrical circuit and is secured to a housing having a cavity in which the transducer is received. The housing is formed with a port for connection to a fluid pressure source to be monitored and a flexible O-ring seated in the metal housing around the port with the transducer biased against the O-ring to form a fluid seal and to define a fluid receiving recess with the diaphragm exposed to fluid in the recess. A fluid could be any substance in liquid or gas form. In that arrangement, the diaphragm is movable in response to variation in pressure applied to the diaphragm to vary the capacitance of the capacitor in accordance with the changes in applied pressure and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
Although sensors of the type described have been very successful, there are certain fluids which are corrosive relative to elastomers used for the O-ring seals. Furthermore, the O-ring seal could be permeable with respect to fluids such as ethanol, methanol and fuels. By way of example, automotive fluids having certain additives are incompatible with typical elastomers. Examples of automotive fluids are ecological fuels like biodiesel and gasoline with ethanol or methanol added.
It is conventional to place a sensing element in a chamber and to seal the chamber with a flexible diaphragm with the chamber filled with relatively incompressible oil to serve as a pressure transfer fluid as shown in U.S. Pat. No. 7,231,830. This approach could be used in sensing a condition, such as pressure, of a fluid to be sensed, which fluid is corrosive to the sensing element. However, this represents a relatively expensive solution in manufacturing a suitable package requiring a number of processing steps including evacuation of a chamber and back filling with oil and providing a seal compatible with the pressure of the sensed fluids.
A known pressure sensor as shown in U.S. Pat. No. 6,272,927, assigned to the assignee of the present invention, the subject matter of which is included herein by this reference, comprises a metal housing having a fluid receiving port leading to a cavity in which a pressure sensing element is received has a flexible metal diaphragm extending over a portion of the cavity which includes the port. The metal diaphragm is hermetically attached to the housing as by welding or brazing. The pressure sensing element having, a ceramic pressure sensing diaphragm is placed in the cavity with the ceramic diaphragm disposed against the metal diaphragm but with a thin intermediate layer of plastic material, preferably somewhat resilient, such as elastomeric material, therebetween. The fluid pressure is transferred to the ceramic diaphragm through the flexible metal diaphragm and intermediate layer with the intermediate layer effectively mitigating hysteresis. However, this represents a relatively expensive solution in manufacturing a suitable package requiring a number of processing steps including welding or brazing the metal diaphragm to the port and positioning the intermediate layer between the metal diaphragm and ceramic pressure sensing diaphragm.